Automatic piercing point change apparatus

ABSTRACT

The invention relates to apparatus for automatically changing and cooling the piercing point used in a piercing mill operation, the point being connected to the piercing bar in a manner so that it may be removed therefrom only after its release from the pierced tube so that it cannot stick in the tube and so that the cooling function may take place outside of the heat range of the mill.

United States Patent Meurer et al.

[ 51 July 24, 1973 AUTOMATIC PIERCING POINT CHANGE APPARATUS Inventors: Hans Meurer; Ewald Bruels, both of Moenchengladbach, Germany Assignee: Wean Industries, Inc., Youngstown,

Ohio

Filed: Sept. 27, 1971 Appl. No.: 184,103

Foreign Application Priority Data Sept. 28, 1970 Germany P 20 48 990.1

US. Cl 72/69, 72/97, 72/201, 72/368 Int. Cl B21b 19/04 Field of Search 72/69, 95-97, 72/345, 346, 364, 367, 368, 370, 200-202, 208, 209; 80/2, 11, 13, 22, 41, 62

References Cited UNITED STATES PATENTS 3/1941 Kelso 72/342 X ///1/7fPA/4L 73425405 Primary Examiner-Charles W. Lanham Assistant Examiner-E. M. Combs Attorney-Michael Williams ABSTRACT The invention relates to apparatus for automatically changing and cooling the piercing point used in a piercing mill operation, the point being connected to the piercing bar in a manner so that it may be removed therefrom only after its release from the pierced tube so that it cannot stick in the tube and so that the cooling function may take place outside of the heat range of the mill.

15 Claims, 7 Drawing Figures PATENTED JUL 2 4 i973 SHEET 1 0F 6 PATENTED 3. 747. 385

saw u or 6 HANs MEURER 33,43 EWALD BRUELS PATENTED 3.747. 385

sum 5 or 6 HAN5 MEURER EWALD BRUELS AUTOMATIC PIERCING POINT CHANGE APPARATUS BACKGROUND AND SUMMARY The apparatus of our invention is particularly suited for use with a Mannesmann piercing mill such as described on pages 741, 742 of the text entitled The Making, Shaping and Treating of Steel Seventh Edition, published by United States Steel Corporation.

In a mill of the type above mentioned, a solid round bar or billet, of proper length and diameter to make the size and weight of the tube desired, is heated uniformly to the usual temperature for rolling light sections. The heated billet is pushed forward into the space between a pair of skewed rotating rolls which have been adjusted so that the pass therebetween is less than the diameter of the billet.

As soon as the leading end of the billet has entered the pass, the force of the pusher is removed. Because of the obliquity of the roll axes, the motion imparted to the billet between the rolls is one of rotation and axial advance. When the leading end of the billet has advanced sufficiently, it encounters the nose or pointed end of the piercer bar. The grip of the rolls is sufficient to continue advance of the workpiece against the retarding effect imposed by the piercer bar. When the rear end of the billet is rolled clear of the piercing point, the thrust bearing carriage supporting the piercer bar is unlatched and the mandrel withdrawn from the billet, which is now a hollow shell.

The piercing point is removably connected to the forward end of the piercer bar and, because of the heat of the billet, must be removed and replaced periodically. Manual changing and cooling of the piercing point, as was done since the introduction of the piercing mill, demanded the utmost strength and concentration of the personnel since the place provided directly at ,the roll saddle exposed the operator not only to the heat and steam, but also to an increased danger of accident. Further, changing of the piercing point when rolling large tube diameters required so much human effort that many attempts have been made to eliminate the manual operation. 7

Such attempts included apparatus for changing and cooling the piercing points wherein certain of the manual work was eliminated, but which still required an operator to push the heated piercing point, after removal from the piercer bar, onto a transfer skid, from where it rolls into a water bath.

Also known to us is apparatus in which the piercing point is connected to the piercer bar by a resilient circle clip and wherein the point is separated from the bar by a vertically movable stop which engages behind the point as the piercer bar is retracted, this action overcoming the elasticity of the clip whereby the separated piercing point may be moved into a water trough by a chain transport. A main disadvantage of this type of connection is that the heat in the rolling area made the durability of the circle clip a questionable factor. Furthermore, it was not possible to pull the piercer bar and attached piercing point back through the rolled tube since the resiliency of the connecting'clip permitted separation of the point from the bar and the point became stuck in the tube. Therefore, the piercing point had to be separated from the bar' before the latter was drawn back through the tube which then required the cooling device to be installed at an insufficient distance from the mill housing and away from the heat of the mill.

We have overcome the foregoing disadvantages of the prior art known to us by disposing a rotatable and axially displaceable cooling pipe within the hollow piercer bar, the pipe having an external thread at its front end which interfits with internal threads on the tail of the piercing point, the latter having a noncircular exterior, such as square, hexagonal, or the like, for sliding fit within a corresponding inner bore in the front end of the bar.

Further, the cooling device comprises a water-filled cooling trough in which a holding drum is turnable by means of a ratchet mechanism, the drum having its lower part immersed in the water of the cooling trough and being formed with a plurality of openings, each of a size to receive a piercing point.

Also, we provide a piercing point setting and holding device in front of the cooling trough, which device comprises a cone-shaped seating mechanism which is matched to the contour of the piercing point and can be turned and displaced from operative position.

Further, we provide fully automatic operation of the changing and cooling of the piercing point through programed sequences which actuate hydraulic and electric operating devices in dependent action.

DESCRIPTION OF THE DRAWINGS In the drawings accompanying this description and forming a part of this specification, there is shown, for

purpose of illustration, an embodiment which our in-- vention may assume, and in these drawings:

FIG. 1 is a side elevational view of apparatus embodying our invention, showing the piercer bar in piercing position,

FIG. 2 is a view similar to FIG. 1 but showing the piercer bar in retracted position,

FIG. 3 is a fragmentary, enlarged sectional view of a portion of the apparatus shown in FIG. 2, and

FIGS. 4 through 7 show the piercing ban piercing point drum and piercing point holding device in various stages of piercing point change.

- DESCRIPTION OF THE PREFERRED EMBODIMENT As seen in FIG. 1, the piercing point 1 tits on the front end of the piercer bar 2 and the latter is rotatably supported in a thrust block 3. In the position shown, the piercing point is adapted to fit in the pass between the skewed rotating rolls of the piercing mill. The rolls are not shown but may be of the type used in a conventional Mannesmann piercing mill, as previously noted. Rotatably mounted on the thrust block 3 are rolls or wheels 4 which roll along a bed 5. A rope or belt 6 is fastened to the bottom side of the thrust block, the rope or belt being trained over rolls 7 and 8 located at opposite ends of the bed 5. One of the rolls 7, 8 is driven in reversible manner by any suitable means, so that the thrust block may be moved to either one of the two positions shown in FIGS. 1 abd 2.

A bridge 9 is provided at the front end of the bed 5, and a fluid cylinder 10 is mounted on the bridge by means of a pivot 11. The piston rod 12 of the cylinder 10 is connected to a lever 13 which is swingable about a pivot 14 carried by the bridge 9. The lever 13 has a clamping surface 15 which matches a corresponding surface 16 on the thrust block 3. In the front part of the bridge 9 is a support or abutment 17 which is cooperable with a lock plate 18 on the thrust block.

A cooling pipe 19 (see also FIG. 3) is disposed within the hollow piercer bar 2, and a turning device 20 for the cooling pipe 19 is mounted on the rear of the bed 5. In front of the bed is a piercing point drum 21 mounted on a cooling trough 22, the drum 21 being cooperable with a counter hold device 23 for fully automatic changing of the piercing point.

A turning devicd is shown in greater detail in FIG. 3., wherein the thrust block 3 is in the retracted position shown in FIG. 2. In such retracted position, the rear end of the cooling pipe 19 is disposed within a clamping chuck 24. The pressure oil for clamping of the chuck is supplied through a pipe 25. By means of screws 26, a gear 27 is mounted to the clamping chuck 24 and this gear meshes with a gear 28. By means of a key 29, the gear 28 is non-rotatably held on the shaft 30 of a motor 31. The gears 27, 28 are mounted in a housing 32 which is closed by a cover 33 held in place by screws 34. The housing 32 is provided with a pair of side bars 35 (only one of which is seen in FIG. 3) and spaced pairs of rolls 36 are mounted on each side bar, the rolls cooperating with respective rails 37 (only one seen in FIG. 3) so that the housing may be moved back and forth along the rails.

Each of the rails 37 has its opposite ends fixed to two brackets 38 and 39, the latter being firmly mounted on the bed 5. The two side bars 35 are connected to the piston rod 40 of a fluid cylinder 41 which is firmly mounted on the bracket 39.

A fluid cylinder 42 is mounted on the tail end of the piercer bar 2 by means of screws 43. In this cylinder slides a piston 44 which is fixed on the cooling pipe 19. The front part of the cylinder has a plurality of slots 45 which are formed in its peripheral wall and are spaced apart thereabout, any one of the slots being engageable with a bolt 46. The bolt is mounted in a guide 47 which is shown as integral with the housing 48. The bolt 46 has a collar 49 on its upper end, and a coil spring 50 is compressed between the collar and the upper end of the guide 47. The collar 49 is in abutment with a piston rod '51 of a fluid cylinder 52, and the latter is secured to the rails 37 by side bars 53. The fluid supply and outlet pipes 54 and 55 lead to cylinder 42 through a manifold ring 56 which has communication with the ports to the cylinder 42.

The housing 48 is fastened to the thrust block 3 by screws 57.'A guide 58 for the cooling pipe 19 is held to the housing 48 by means of screws 59. The cooling water enters the cooling pipe 19 through a supply pipe 60, an annular chamber 61 within the guide 58, and an opening 62 in the wall of the cooling pipe 19. The return flow of water is through the annular space between the interior of the piercer bar 2 and the exterior of the cooling pipe 19, through an opening 63 in the wall of the piercer bar 2, through housing 48 and drain conduit 64.

As best seen in FIGS. 4 through 7, an external thread 65 is formed on the front end of the cooling pipe 19, and this thread engages with internal threads 66 formed at the tail end of the piercing point 1. A female internal seat 67 of non-circular cross-section, preferably square, hexagonal, octagonal or the like, is formed at the front end of the piercer bar 2, and this seat receives a mating external contour 68 formed on the tail end of the piercing point 1.

The cooling drum 21, in the present embodiment, is shown as a pair of spaced plates having circularly disposed pairs of aligned openings 69, each pair being adapted to receive and support a piercing point 1. The plates are secured to a hub which is fixed to a shaft 70, the latter being rotatable in bearings 71 which are mounted on the upper rim of the cooling trough 22. At one end of the shaft is a ratchet 72 from which a lever 73 extends. The lever 73 is connected, by bolt 74, to the clevis end 75 of a piston rod extending from a fluid cylinder 76. The stroke of the piston rod in one direction will swing the lever 73 to cause operation of the ratchet 72 and corresponding rotation of the shaft 70, whereas in the reverse movement of the piston rod the ratchet will free wheel and therefore it will not effect rotation of the shaft 70.

A seating member 77 is provided for holding the piercing point 1 when it is screwed in or out of the cool ing pipe 19. The member 77 has a cone-shaped recess in which the front end of the piercing point is adapted to seat. The member 77 is mounted on the outer end of a piston rod 78 which is part of the fluid cylinder 79. A pair of levers 80 are fixed to and extend from the cylinder 79, the free end of the levers 80 being fixed to a shaft 81 which is rotatable in bearings 82 carried at the upper end of a suitable frame 88. A lever 83 has one end fixed to the shaft and its opposite end connected, by a bolt 84, to a clevis 85 formed on a piston rod 86. The piston rod is part of a fluid cylinder 87 which has its lower end pivoted to the frame 88 by means of a bolt 89.

The method of operation of the automatic piercing point changing and cooling device, as actuated by proper programing, is as follows:

As shown in FIG. 1, the piercing point 1, piercer bar 2 and thrust block 3 are in working position wherein the piercing point is disposed relative to the skewed rolls of the piercing mill for engagement with the billet. The movable thrust block is locked in such working position within the bridge 9 by the lever 13, while the piercing point I, screwed onto the cooling pipe 19, is held in tension by means of piston 44of cylinder 42. Such tension is created byfluid pressure within the cylinder 42 in advance of the piston 44, and causes the cooling pipe to be urged rearwardly, thereby holding the shoulder formed by the male seat 68 at the tail end of the piercing point 1 in abutment with the forward end of the piercer bar to prevent withdrawal of the male seat 68 from the female seat 67 at the front end of the piercer bar. Tension on the cooling pipe also prevents any whipping action thereof during rotation.

After the rolling process has been completed on the billet to form the resultant tube, the thrust block 3 is unlocked from the bridge 9 by admitting fluid pressure to the cylinder 10 to cause the piston rod 12 to retract and lift the lever 13, as seen in FIG. 2. After the thrust block 3 has been unlocked, the drive for one of the wheels 7 is programed to cause the rope or belt drive 6 to draw the thrust block rearwardly to its position shown in FIG. 2.

Rearward movement of the thrust block 3 withdraws the piercer bar 2 through the formed tube, which is held against axial movement in any suitable manner, and through the upper pair of aligned openings 69 in the cooling drum plates, and the rearward movement of the thrust block 'is of such length that-the piercing point comes to disposition within such aligned openings, as seen in FIGS. 2 and 5. At a suitable time the programer causes pressure fluid to flow to the cylinder 87 so that the piston rod 86, through levers 80, swings the seating member 77 to axially align it with the axis of the piercer bar 2. ln the rearward position of the thrust block, the rear end of the cooling pipe is entered into the clamping chuck 24, at which time the programer causes fluid under pressure to flow through pipe 25 and set the jaws of the chuck on the cooling pipe. Rearward movement of the thrust block to the position shown in FIG. 2 also causes the collar 49 on the bolt 46 to align axially with the piston rod 51 of cylinder 52.

When the cooling pipe is firmly clamped in the chuck 24, the programer energizes the electric motor 31 which drives meshing gears 27, 28, and since gear 27 is connected to the chuck, the latter is rotated and in turn rotates the cooling pipe. Rotation imparted by motor 31 is in such direction as to rotate cooling pipe in a direction to unthread the connection between the cooling pipe 19 and the piercing point 1. During this time the piercer bar 2 is secured against rotation by reason of the fact that the programer has caused pressure fluid in the cylinder 52 to project the piston rod 51 which in turn moves the bolt 46 so that its inner end engages in one of the slots 45 on the peripheral surface of the cylinder 42, the latter it being remembered, having connection with the piercing bar 2 by means of the screws 43. During this time, pressure fluid in the cylinder 42 is exhausted so that the piston 44 of this cylinder does not effect any tension on the cooling pipe 19.

FIG. 5 shows the position of parts just prior to the time that the cooling pipe 19 is rotated to unscrew it from the piercing point 1, and FIG. 6 shows the parts in position after the unscrewing operation. It will be noted that the unscrewing operation causes the piercing point 1 to be moved axially and forwardly of the piercer bar 2 so that the male seat 68 on the piercer bar has been removed from the female seat 67 in the piercing bar 2. The seating member 77 at this time has been moved toward the piercing point to hold the latter steady and within the upper pair of aligned openings in the cooling drum 21. After the unscrewing operation the cooling pipe is urged rearwardly by means of the fluid cylinder 4l.acting through the side bars 35 which are connected to the housing for the gears 32, 33. This movement insures clearance for rotation of the drum 21.

The cylinder 79 is then actuated to withdraw the seating member 77 to its position shown in FIG. 7. The removed piercing point 1 now rests freely in the upper aligned openings 69 in the piercing point drum 21, whereupon the cylinder 76 is actuated to cause the ratchet 72 to turn the drum a sufficient amount so that the just-removed piercing point 1 is moved into the water in the cooling tank 22.

As seen in the drawings, the cooling drum 21 supports several piercing points, and as the just-removed piercing point is moved to the water, another piercing point is moved to the position shown in FIG. 7 for connection to the cooling pipe 19 and piercer bar 2. To effect such connection, the seating member 77 is moved to the position shown in FIG. 6. The nose of the point will seat in the cone-shaped recess of the member and will thereby be axially aligned with the piercing bar 2 and cooling pipe 19, and the male seat 68 on the piercing point will be ready for entrance into the female seat 67 in the piercing bar 2. If the flats of the two seats are not in alignment they will be aligned as soon as the cooling pipe is rotated.

The cooling pipe 19 is rotated in the opposite direction of its unthreading rotation by reason of the fact that the electric motor 31 is reversed in its rotation and thereby imparts the proper rotation to the cooling pipe through gears 27, 28 and clamping chuck 24. At the same time pressure fluid is entered into cylinder 41 to move the side bars 35, the housing for the gears 27, 28 and the clamping chuck 24 and cooling pipe forwardly, while the piercer bar 2 is still held against rotation by the bolt 46.

After the screw connection between piercing point 1 and cooling pipe 19 is made, the clamping chuck 24 is disengaged from the cooling pipe and the seating member 77 swung out of the way to its position shown in FIG. 4. The thrust block 3 is then moved to its position within the bridge 9, as seen in FIG. 1, and is locked in such position by the lever 13. It will be noted that as the thrust block 3 moves forwardly, the piercer bar 2 moves through the upper pair of aligned openings 69 in the cooling drum 21, and therefore this drum cannot rotate. Prior to forward movement of the thrust block, the piston rod 51 of cylinder is retracted so that spring 50 will retractbolt 46 from engagement with slot 45.

In the forward locked position of the thrust block 3, the piercing point is positioned in proper relation with the skewed rolls of the piercing mill and the billet which is advanced by the rolls. Pressure fluid is again introduced into cylinder 42 in advance of piston 44 so as to place tension on the cooling pipe.

The apparatus of our invention assures a fully automated method for changing and cooling piercing points and eliminates the manual labor heretofore required. Further, the activation of all cylinders and motors maybe effected in proper sequence by a programer. The connection between the piercing point and cooling pipe, through the screw connections 65, 66, assures that the piercing point will be adequately held to the piercer bar underany circumstances. The fact that the piston 44, activated by pressure fluid in the cylinder 42, may yield, and the male and female seats 67, 68 on the piercer bar 2 and piercing point 1 may slide relatively, provides for an automatic adjustment for all differences of length which are caused by thermal expansion and contraction, or manufacturing tolerances.

Our invention therefore permits the cooling trough 22 to be arranged at some distance from the mill and enables the guiding of the piercer bar close to the mill housing. Further, the invention makes it possible to arrange the drive shafts for the skewed rolls on the outlet side.

We claim:

1. For use with a piercing mill having rolls for rotating and advancing a billet, an elongated piercer bar, a piercing point having one end connected to an end of said piercer bar to extend axially thereof, the connection comprising an axially extending non-circular socket in said piercer bar and an axially extending non-- circular projection on said piercing point slidably fitting within and mating with said socket to hold said piercing point against rotation relative to said piercing bar, and a cooling pipe disposed axially within said tubular piercer bar and having one end connected to said piercing point one end by longitudinally extending screw threads, the sliding fit of said projection and socket providing for longitudinal separation of said piercing point and piercer bar when the latter is held against rotation and said cooling tube and piercing point are unthreaded to separate the same.

2. The construction according to claim 16 wherein said one end of said piercing point is formed with a non-circular projection which mates and slidably fits within a non-circular socket formed in said end of the piercer bar, and wherein said one end of said cooling pipe is formed with external screw threads which engage internal screw threads formed in said one end of said piercing point.

3. The construction according to claim 2 wherein a piston is connected to the opposite end of said cooling pipe, said piston being disposed in a cylinder which is fixed to the opposite end of said piercer bar, and means for introducing pressure fluid in to said cylinder to urge said piston and the cooling pipe connected thereto in a direction to pull on said piercing point and thereby place said cooling pipe in tension.

4. The construction according to claim 2 and including means for rotating said cooling pipe in a direction to unscrew the threaded connection between said cooling pipe and said piercing point, and means for holding said piercer bar against rotation during the unscrewing operation.

5. The construction according to claim 2 wherein said piercer bar is supported in cantilever manner from a thrust block, and wherein said thrust block is movable along a bed to advanced or retracted positions, in advanced position said thrust block being locked in a bridge secured to said bed with the piercing point in operative relation to the billet being advanced by said rolls, and in retracted position said thrust block disposing said piercing point in juxtaposition to cooling means.

6. The construction according to claim 5 and further including a drum rotatable on an axis parallel to the piercer bar axis and having an off-center opening through which said piercer bar moves and which is of a size to receive and support said piercing point, said drum being related to said thrust block when the latter is in retracted position so that said piercer bar is fully retracted from said off-center opening and said piercing point is disposed within said opening, means for rotating said cooling pipe in a predetermined direction and means for holding said piercing bar against rotation whereby the threaded connection between said piercing point and said cooling pipe .is separated, leaving said piercing point supported in said off-center opening, and means for rotating said drum to move said piercing point to said cooling means.

7. The construction according to claim 6 wherein said drum is formed with another similar off-center opening, and another similar piercing point supported in the latter opening, rotation of said drum removing the first named piercing point from the axis of said piercing bar anddelivering said other piercing point to said axis for connection to said piercer bar and cooling pipe.

8. The construction according to claim 4, wherein said piercing point has a conical nose portion, and further including a seating member having a cone-shaped opening complementary to said nose portion, said seating member being movable from an inoperative position clear of the axis of movement of said piercer bar to an operative position on said axis and in the latter 8 position being movable axially to dispose its opening over and the opening wall in supporting relation with respect to said nose portion.

9. Apparatus for use with a piercing mill having rolls for rotating and advancing a billet, said apparatus comprising an elongated bed, a bridge at one end of said bed, a thrust block movable along said bed between two positions, in one position being locked in advanced relation in said bridge against movement along said bed and in another position being disposed in retracted relation at the other end of said bed, and means for selectively moving said thrust block from one position to another, the improvement comprising:

an elongated tubular piercer bar having one end rotatably mounted in said thrust block and extending in cantilever fashion therefrom along an axis parallel to the line of movement of said thrust block, the opening at the forward free end of said piercer bar being non-circular,

a cooling pipe within said piercer bar, having an ex ternally threaded forward end adjacent to the forward end of said piercer bar,

a piercing point having a non-circular projection at its tail portion which is complementary to and slidably fits within the non-circular opening in said piercer bar, and also having an internally threaded opening into which is threaded the forward end of said cooling pipe,

said piercing point being held against axial movement and disposed in operable relation with said billet rolls when said thrust block is locked in said bridge so that the billet advanced by the rolls is pieced by said piercing point to progressively form a tube which moves axially over said piercing point and said piercer bar, and said piercer bar and piercing point being pulled through the formed tube as said thrust block is moved to retracted position.

10. The construction according to claim 9 wherein a piston is connected to the rear portion of said cooling pipe, said pistonbeing disposed in a cylinder which is fixed to said piercer bar one end, and means for introducing pressure fluid into said cylinder to urge said piston and the cooling pipe connected thereto in a rearward direction to pull-on said piercing point andthereby place said cooling pipe in tension.

11. The construction according to claim 9 and further including means for rotating said cooling pipe in a direction to unscrew the threaded connection between said cooling pipe and said piercing point, and means for holding said piercer bar against rotation during the unscrewing operation.

12. The construction according to claim 9 wherein the rear end of said cooling pipe extends outwardly of said piercer bar one end, and a clamping chuck supported at said bed other end and adapted to receive and hold said cooling pipe rear end when said thrust block is in retracted relation, and means for rotating said chuck when said cooling pipe rear end is held therein to unscrew the threaded connection between said cooling pipe and said piercing point, and means for holding said piercer bar against rotation during the unscrewing operation.

13. The construction according to claim 9 and further including a drum rotatable on an axis parallel to the piercer bar axis and having an off-center opening through which said piercer bar moves and which is of a size to receive and support said piercing point, said drum being related to said thrust block when the latter is in fully retracted relation so that said piercer fully retracted from said off-center opening and said piercing point is disposed within said opening, means for rotating said cooling pipe in a predetermined direction, and means for holding said piercer bar against rotation whereby the threaded connection between said piercing point and said cooling pipe is separated, leaving said piercing point supported in said off-center opening, and means for rotating said drum to move said piercing point in juxtaposition to cooling means.

14. The construction according to claim 13 wherein said drum is formed with another similar off-center the first named piercing point from the axis of said piercing bar and delivering said other piercing point to said axis for connection to said piercer bar and cooling pipe.

15. The construction according to claim 11 and further including a seating member having a cone-shaped opening complementary to the nose portion of said piercing point, said seating member being movable from an inoperative position clear of the axis of movement of said piercing bar to an operative position on said axis and in the latter position being movable axially to dispose its opening over and the opening wall in supporting relation with respect to said nose portion. 

1. For use with a piercing mill having rolls for rotating and advancing a billet, an elongated piercer bar, a piercing point having one end connected to an end of said piercer bar to extend axially thereof, the connection comprising an axially extending non-circular socket in said piercer bar and an axially extending non-circular projection on said piercing point slidably fitting within and mating with said socket to hold said piercing point against rotation relative to said piercing bar, and a cooling pipe disposed axially within said tubular piercer bar and having one end connected to said piercing point one end by longitudinally extending screw threads, the sliding fit of said projection and socket providing for longitudinal separation of said piercing point and piercer bar when the latter is held against rotation and said cooling tube and piercing point are unthreaded to separate the same.
 2. The construction according to claim 16 wherein said one end of said piercing point is formed with a non-circular projection which mates and slidably fits within a non-circular socket formed in said end of the piercer bar, and wherein said one end of said cooling pipe is formed with external screw threads which engage internal screw threads formed in said one end of said piercing point.
 3. The construction according to claim 2 wherein a piston is connected to the opposite end of said cooling pipe, said piston being disposed in a cylinder which is fixed to the opposite end of said piercer bar, and means for introducing pressure fluid in to said cylinder to urge said piston and the cooling pipe connected thereto in a direction to pull on said piercing point and thereby place said cooling pipe in tension.
 4. The construction according to claim 2 and including means for rotating said cooling pipe in a direction to unscrew the threaded connection between said cooling pipe and said piercing point, and means for holding said piercer bar against rotation during the unscrewing operation.
 5. The construction according to claim 2 wherein said piercer bar is supported in cantilever manner from a thrust block, and wherein said thrust block is movable along a bed to advanced or retracted positions, in advanced position said thrust block being locked in a bridge secured to said bed with the piercing point in operative relation to the billet being advanced by said rolls, and in retracted position said thrust block disposing said piercing point in juxtaposition to cooling means.
 6. The construction according to claim 5 and further including a drum rotatable on an axis parallel to the piercer bar axis and having an off-center opening through which said piercer bar moves and which is of a size to receive and support said piercing point, said drum being related to said thrust block when the latter is in retracted position so that said piercer bar is fully retracted from said off-center opEning and said piercing point is disposed within said opening, means for rotating said cooling pipe in a predetermined direction and means for holding said piercing bar against rotation whereby the threaded connection between said piercing point and said cooling pipe is separated, leaving said piercing point supported in said off-center opening, and means for rotating said drum to move said piercing point to said cooling means.
 7. The construction according to claim 6 wherein said drum is formed with another similar off-center opening, and another similar piercing point supported in the latter opening, rotation of said drum removing the first named piercing point from the axis of said piercing bar and delivering said other piercing point to said axis for connection to said piercer bar and cooling pipe.
 8. The construction according to claim 4, wherein said piercing point has a conical nose portion, and further including a seating member having a cone-shaped opening complementary to said nose portion, said seating member being movable from an inoperative position clear of the axis of movement of said piercer bar to an operative position on said axis and in the latter position being movable axially to dispose its opening over and the opening wall in supporting relation with respect to said nose portion.
 9. Apparatus for use with a piercing mill having rolls for rotating and advancing a billet, said apparatus comprising an elongated bed, a bridge at one end of said bed, a thrust block movable along said bed between two positions, in one position being locked in advanced relation in said bridge against movement along said bed and in another position being disposed in retracted relation at the other end of said bed, and means for selectively moving said thrust block from one position to another, the improvement comprising: an elongated tubular piercer bar having one end rotatably mounted in said thrust block and extending in cantilever fashion therefrom along an axis parallel to the line of movement of said thrust block, the opening at the forward free end of said piercer bar being non-circular, a cooling pipe within said piercer bar, having an externally threaded forward end adjacent to the forward end of said piercer bar, a piercing point having a non-circular projection at its tail portion which is complementary to and slidably fits within the non-circular opening in said piercer bar, and also having an internally threaded opening into which is threaded the forward end of said cooling pipe, said piercing point being held against axial movement and disposed in operable relation with said billet rolls when said thrust block is locked in said bridge so that the billet advanced by the rolls is pieced by said piercing point to progressively form a tube which moves axially over said piercing point and said piercer bar, and said piercer bar and piercing point being pulled through the formed tube as said thrust block is moved to retracted position.
 10. The construction according to claim 9 wherein a piston is connected to the rear portion of said cooling pipe, said piston being disposed in a cylinder which is fixed to said piercer bar one end, and means for introducing pressure fluid into said cylinder to urge said piston and the cooling pipe connected thereto in a rearward direction to pull on said piercing point and thereby place said cooling pipe in tension.
 11. The construction according to claim 9 and further including means for rotating said cooling pipe in a direction to unscrew the threaded connection between said cooling pipe and said piercing point, and means for holding said piercer bar against rotation during the unscrewing operation.
 12. The construction according to claim 9 wherein the rear end of said cooling pipe extends outwardly of said piercer bar one end, and a clamping chuck supported at said bed other end and adapted to receive and hold said cooling pipe rear end when said thrust block is in retracted relation, and means for rotatinG said chuck when said cooling pipe rear end is held therein to unscrew the threaded connection between said cooling pipe and said piercing point, and means for holding said piercer bar against rotation during the unscrewing operation.
 13. The construction according to claim 9 and further including a drum rotatable on an axis parallel to the piercer bar axis and having an off-center opening through which said piercer bar moves and which is of a size to receive and support said piercing point, said drum being related to said thrust block when the latter is in fully retracted relation so that said piercer fully retracted from said off-center opening and said piercing point is disposed within said opening, means for rotating said cooling pipe in a predetermined direction, and means for holding said piercer bar against rotation whereby the threaded connection between said piercing point and said cooling pipe is separated, leaving said piercing point supported in said off-center opening, and means for rotating said drum to move said piercing point in juxtaposition to cooling means.
 14. The construction according to claim 13 wherein said drum is formed with another similar off-center opening, and another similar piercing point supported in the latter opening, rotation of said drum removing the first named piercing point from the axis of said piercing bar and delivering said other piercing point to said axis for connection to said piercer bar and cooling pipe.
 15. The construction according to claim 11 and further including a seating member having a cone-shaped opening complementary to the nose portion of said piercing point, said seating member being movable from an inoperative position clear of the axis of movement of said piercing bar to an operative position on said axis and in the latter position being movable axially to dispose its opening over and the opening wall in supporting relation with respect to said nose portion. 